Gravimetric measurement of latex solids fraction as a function of drying time during film formation was used as a rough indicator of the point of particle close packing. Parallel measurements of the wet film’s pH with indicator paper revealed an increase in acidity over time, reaching a pH of 6 after 6 min of drying. In a complementary experiment, litmus was added to the wet latex dispersion prior to film formation. This experiment clearly revealed that the film became significantly acidic when dried. The decreasing pH during film formation is attributed to the loss of ammonia through evaporation. Significantly, latex preneutralized with nonvolatile amine did not exhibit the pH change.